It is well known that high value articles are prone to counterfeiting. With improvements in technology and counterfeiter skill, it has become increasingly difficult to visually distinguish many counterfeit goods from their genuine counterparts.
Numerous steps have been taken to allow genuine articles to be authenticated. For example, unique batch numbers, holograms, or bar codes may be printed or adhered to the articles. Additionally, ‘track and trace’ technology may be employed, where a product has a unique code, which enables its current and past locations, as well as other information, to be determined.
One drawback associated with the use of these visible security features is that counterfeiters will often seek to either copy or generate batch numbers or codes which appear, at least at first glance, to be authentic.
It would be desirable to provide a method which enables the rapid and simple authentication of articles using features which are not readily apparent to a counterfeiter and which, even if identified, cannot be readily reproduced.
We have addressed previously issues concerning the ability to authenticate polymeric films as security documents, bank notes and the like in our WO 2009/133390, which discloses a method for authenticating a polymer film by white light interferometry or by birefringence. Whilst this disclosure also contemplates the authentication of objects comprising polymeric films, for example as wrappers of or attachments to an item, it is not contemplated in this document to provide means for the authentication of articles whilst still marked in some way (for example by means of a wrapper or an attachment) with the film. In other words, WO 2009/133390 contemplates only authentication of the films themselves or objects such as banknotes and the like comprising the films, or objects such as wrappers of or attachments to an item after removal from the item; and not of articles themselves marked with the films. Consequently, WO 2009/133390 fails to recognise that the surface of an article marked with a film may itself form a reflective surface from which light may reflect back from a light source and towards a detector. WO 2009/133390 only contemplates that the film itself, or a layer therein, may form a reflective surface.
WO 2007/072426 discloses a method of manufacturing a polarization retardant film and to the use of such a film in security applications.
US 2006/0187452 discloses methods for determining the birefringence level of optical material such as polymeric film.
U.S. Pat. No. 5,737,298 discloses a technique for verifying the authenticity of a particular breed of pirated optical discs using a polarimeter.
JP 2005254643 discloses a multilayer transparent film having a blue shift property in which film of two different refractive indices is sequentially laminated.
US 2005/0109984 discloses a method for authenticating that a test polymer is an authenticatable polymer using a thermochromic compound.
WO 2005/086099 discloses a currency genuineness detection system using a plurality of opto-electronic sensors with both transmission and reflectance properties.